Image forming optical system and electronic instrument using the same

ABSTRACT

An image forming optical system comprises, in order from an object side, a first positive meniscus lens having a convex surface directed toward an object side, an aperture stop, a second positive meniscus lens having a convex surface directed toward the object side and a negative lens. At least one of surfaces of the negative lens is a spherical and the following condition is satisfied: 
     −2.0&lt;Φ m/Φp &lt;0 
     −2.0&lt;( r 1 r+r 2 f )/( r 1 r−r 2 f )&lt;1.0 
     where Φm represents the power of the negative lens at the position of the maximum light height and Φp represents the power of the negative lens at the position of the near axis,  r 1 r  represents the radius of curvature of the first lens at the image side and  r 2 f  is the radius of curvature of the second lens at the object side.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image forming optical systemwhich can be used for an imaging unit with the solid-state imagingelement of CCD, CMOS and the like. For example, it relates to an imageforming optical system which can be used for a miniature camera and amonitor camera and the like which are equipped in, for example, adigital still camera, a digital video camera, a cellular phone, PC andthe like.

[0003] The present invention also relates to an electronic instrumentsuch as a digital still camera, a digital video camera, a cellularphone, PC and the like which use the image forming optical system.

[0004] 2. Description of the Related Art

[0005] In recent years, electronic cameras for taking a photograph byusing a solid-state imaging element like CCD and CMOS instead of usingsilver salt film have become popular.

[0006] In such electronic cameras, for an imaging unit which is equippedin a portable type computer or a cellular phone and the like,miniaturization and weight-lightening have been particularly demanded.

[0007] As an image forming optical system used for such imaging unit,conventionally there are systems constituted with single lens or twolenses. However, it has been already known that in the image formingoptical system having such constitution as mentioned above, curvature offield cannot be corrected and high performance cannot be expected.Therefore, it is necessary to constitute with three or more lenses arerequired in order to achieve high performance in an image formingoptical system.

[0008] On the other hand, when an imaging element like CCD is used foran imaging unit, if off-axis luminous flux emanated from an imageforming optical system enters with too large incident angle to an imageplane, condensing performance of a micro lens is not fully exercised.Accordingly, problem that the brightness of a picture image changesextremely in the central portion and the circumferential portion of thepicture image arises.

[0009] Therefore, the incident angles of the light to an imaging elementas CCD or the like, that is, the position of an exit pupil becomesimportant in designing. Furthermore, the position of an aperture stopbecomes important when an optical system is constituted with smallnumber of lenses.

[0010] As systems in which these problems are taken into consideration,there are optical systems where an aperture stop is arranged from anobject side before the first lens, or between the first lens and thesecond lens. As conventional examples, such image forming opticalsystems have been disclosed in the following documents such as Japanesepublished unexamined patent applications: Toku Kai Hei 05-188284, TokuKai Hei 07-27974, Toku Kai Hei 09-288235, Toku Kai Hei 11-52227 and TokuKai 2001-83409.

SUMMARY OF THE INVENTION

[0011] The image forming optical system of the present inventioncomprises, in order from an object side, a first lens which is meniscuslens having positive refractive power and a convex surface directedtoward an object side, an aperture stop, a second lens which is meniscuslens having positive refractive power and a convex surface directedtoward the object side, and a third lens having negative refractivepower.

[0012] The electric device according to the present invention isequipped with the image forming optical system mentioned above.

[0013] According to the present invention, an image forming opticalsystem in which degradation of performance to manufacture error islittle and high performance is achieved whenever it is miniaturized canbe offered.

[0014] These and other features and advantages of the present inventionwill become apparent from the following detailed description of thepreferred embodiments when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a sectional view showing an optical arrangementdeveloped along the optical axis in the first embodiment of an imageforming optical system according to the present invention.

[0016]FIG. 2A, 2B and 2C are graphs showing spherical aberration,astigmatism and distortion of an image forming optical system in thefirst embodiment respectively.

[0017]FIG. 3 is a sectional view showing an optical arrangement,developed along the optical axis in the second embodiment of an imageforming optical system according to the present invention.

[0018]FIG. 4A, 4B and 4C are graphs showing spherical aberration,astigmatism and distortion of an image forming optical system in thesecond embodiment respectively.

[0019]FIG. 5 is a sectional view showing an optical arrangement,developed along the optical axis in the third embodiment of an imageforming optical system according to the present invention.

[0020]FIG. 6A, 6B and 6C are graphs showing spherical aberration,astigmatism and distortion of an image forming optical system in thethird embodiment respectively.

[0021]FIG. 7 is a sectional view showing an optical arrangement,developed along the optical axis in the fourth embodiment of an imageforming optical system according to the present invention.

[0022]FIG. 8A, 8B and 8C are graphs showing a spherical aberration, anastigmatism and a distortion of an image forming optical system in thefourth embodiment respectively.

[0023]FIG. 9A and 9B are a front view and a rear view showing anoutlined construction of a cellular phone using an image forming opticalsystem according to the present invention.

[0024]FIG. 10A and 10B are a front perspective view and a rearperspective view showing an outlined construction of a digital camerausing an image forming optical system according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Prior to explaining embodiments, reasons why the constitution ofthe present invention has been made as well as function and advantagesof the present invention will be explained.

[0026] First of all, explanation is made with respect to the number oflenses constituting an image forming optical system.

[0027] Taking into consideration of performance and miniaturization, inthe present invention, an image forming optical system is constitutedwith three lenses.

[0028] If an image forming optical system is constituted using four ormore lenses, it is evident that the performance is improved further.

[0029] However, when the number of lens increases one by one, thethickness of lenses, the distance between lenses and the space of frameincrease accordingly and enlarging of the size is inevitable.

[0030] In case that an image forming optical system is constituted withtwo lenses or one lens as adopted in the conventional technology asmentioned above, the performance of circumference deterioratesconsiderably as the curvature of the image plane can not become small.

[0031] On the other hand, if three lenses are used in an image formingoptical system, the performance and the size becomes optimal.

[0032] Next, when an imaging element like CCD is used in an unit usingan image forming optical system, it is desirable that an aperture stopis arranged at the position distant from the image plane.

[0033] By such arrangement, incident angle of the light to the imagingelement can be made small for keeping good performance of condensinglight.

[0034] On the other hand, in an optical system having wide angle ofview, it is desirable that the aperture stop is arranged symmetricallyto power arrangement of the optical system.

[0035] Thereby, generation of distortion of circumferential portion ofthe picture plane and magnification chromatic aberration can be reduced.

[0036] From two reasons mentioned above, in the image forming opticalsystem the position of the aperture stop is set between the first lensand the second lens.

[0037] By this arrangement, an optical system in which telecentricfunction is much noted has been realized.

[0038] This enable to realize most effectively that incident angle ofthe light to the image element is small as well as generation of thedistortion of circumferential portion of the picture plane andmagnification chromatic aberration are reduced.

[0039] Moreover, in the image forming optical system, if the first lensis constituted with a meniscus lens having positive power and a surfacewith strong curvature toward an object side, the principal pointposition of the first lens having positive power can be moved to theobject side, and it become advantageous for shortening the whole length.

[0040] In addition, in the image forming optical system, the first lensand the second lens are a meniscus lens having positive power and aconvex surface directed toward the object side and a meniscus lenshaving positive power and a convex surface directed toward the imageside respectively, and an aperture stop is placed between the first lensand the second lens.

[0041] By this way, the power arrangement in the optical viewpointbecomes, in order from the object side, (positive, negative) and(negative, positive).

[0042] As a result, deflection angle, that is, an angle which is formedby an incident light and an exit light, can be kept small, andgeneration of aberration can be suppressed.

[0043] As the generation of aberration in case of non-decentering issmall by nature, it is the constitution which is hard to be influencedby the performance fluctuation in case of relative decentering of alens.

[0044] As mentioned above, in the image forming optical system the firstlens has positive power and the second lens has negative power as apower arrangement in order to shorten the whole length of an opticalsystem.

[0045] However, in a wide angle optical system, if a lens which is atthe utmost image side has negative power from the center portion to thecircumferential portion the following inconvenience occurs. For example,it is supposed that CCD, as an imaging element, which limits incidentangle is used in order to avoid a shading.

[0046] In this case, if the power is negative from the center portion tothe circumferential portion, the incident angle of the light cannot bemade small at the position where the height of the light is large.

[0047] Then, at least one of surfaces of the lens which is at the utmostof an image plane side is made aspherical and the power of thecircumferential portion of the lens is made positive, even if the powerof the center portion of the lens is negative.

[0048] In this constitution, by refracting the light, at the positionwhere the height of the light is high, widely toward the optical axisside, the incident angle of the light to the image plane can be small.

[0049] Therefore, in the image forming optical system, it is importantthat the following condition (1) is satisfied in the third lens which isat the utmost image side.

−2.0<Φm/Φp<0  (1)

[0050] where Φm is the power of the third lens at position with themaximum height of the light and Φp is the power of the third lens on theparaxis.

[0051] Here, the power Φm of the lens in the position with maximumheight of the light is defined as follows. It is given by Φm=tan ξ/Hm,when a parallel light is entered to the maximum height Hm of the lightof the lens that is an object from the infinite point of the objectside, and an inclined angle after passing through the lens isrepresented by ξ.

[0052] When Φm/Φp falls below the minimum limit of this condition (1),the power of the paraxis becomes weak too much and the whole length hasbecome long, otherwise, the positive power of circumference becomesstrong too much. Thus, the performance of circumference is remarkablydegraded. On the other hand, when it exceeds the upper limit, thepositive power of circumference of the lens becomes weak too much andthe correction of the incident angle of the light to the image planebecomes insufficient.

[0053] It is better to satisfy the following condition (1′).

−1.0<Φm/Φp<0  (1′)

[0054] Further, it is much better to satisfy the following condition(1″).

−0.5<Φm/Φp<0  (1″)

[0055] Furthermore, in the image forming optical system it is betterthat the following condition(2) is satisfied with respect to the radiusof curvature of the surface at the image side of the first lens and theradius of curvature of the surface at the object side of the secondlens.

0<(r 1 r+r 2 f)/(r 1 r−r 2 f)<1.0  (2)

[0056] where r1r is the radius of curvature of the surface at the imageside of the first lens and r2f is the radius of curvature of the surfaceat the object side of the second lens.

[0057] If condition (2) is satisfied, the deflection angle of the firstlens and the second lens can be made small, and it becomes possible tosuppress the performance degradation by the relative decentering oflenses.

[0058] When (r1r+r2f)/(r1r−r2f) exceeds the upper limit of the condition(2), the negative power at the object side of the second lens becomessmall too much and an aberration generated in the first lens cannot bewell corrected. On the other hand, when it falls below the minimum limitof the condition (2), the negative power at the object side of thesecond lens becomes strong too much. As a result, it becomes difficultto correct spherical aberration and comatic aberration generated in thissurface by using other surfaces.

[0059] Preferably it is desirable that in the image forming opticalsystem the following condition (2′) is satisfied.

0.2<(r 1 r+r 2 f)/(r 1 r−r 2 f)<0.9  (2′)

[0060] Furthermore, it is desirable that in the image forming opticalsystem the following condition (2″) is satisfied.

0.5<(r 1 r+r 2 f)/(r 1 r−r 2 f)<0.7  (2″)

[0061] In an image forming optical system, in order to shorten the wholelength of an image forming optical system it is necessary to arrange theprincipal point position of the image forming optical system near to theobject side. Accordingly the power of the first lens is important.

[0062] Therefore, in the image forming optical system, it is desirableto satisfy the following condition (3).

0.1<r 1 f/f<1.0  (3)

[0063] where r1f is the radius of curvature of the surface at the objectside of the first lens, and f is the whole focal length of the imageforming optical system.

[0064] When r1f/f exceeds the upper limit of the condition (3), theradius of curvature of the first surface becomes large and the principalpoint position of the first lens having positive power becomes at theimage side.

[0065] In order to shorten the whole length of the image forming opticalsystem, the power of each lens must be strengthen. However, if the poweris strong an aberration is easily generated. Accordingly it becomesdifficult to obtain a predetermined performance.

[0066] On the other hand, when it falls below the minimum limit of thecondition (3), it is advantageous to shorten the whole length of animage forming optical system, but correction of the spherical aberrationgenerated by the first surface becomes difficult.

[0067] In the image forming optical system, it is preferable to satisfythe following condition (3′).

0.2<r 1 f/f<0.8  (3′)

[0068] Furthermore, in the image forming optical system it is preferableto satisfy the following condition (3″).

0.3<r 1 f/f<0.6  (3″)

[0069] The following constitution is adopted in the image formingoptical system in order to shorten the whole length.

[0070] That is, it is a telephoto type optical system by the positivepower in the first lens and the second lens, and the negative power inthe third lens.

[0071] Therefore, in the arrangement of this telephoto type havingpositive power and negative power, in order to attain shortening of thewhole length as well as securing of performance with good balance in theimage forming optical system, it is desirable to satisfy the followingconditions (4) and (5).

0.2<f 12/|f 3|<1.5  (4)

0.5<f/|f 3|<2.0  (5)

[0072] where f12 is total focal length of the first lens and the secondlens, f3 is focal length of the third lens and f is the whole focallength of the image forming optical system as a whole system.

[0073] If the conditions (4) and (5) are not satisfied, the balance ofthe positive power and negative power which constitute the telephototype is collapsed and the whole length of the image forming opticalsystem increases or the performance is degraded. That is, when exceedingthe upper limit of the conditions (4) and (5), the negative power whichconstitutes the telephoto type becomes weak and therefore it becomesdisadvantageous for shortening of the whole length of the image formingoptical system. On the other hand, when falling below the minimum limitof the conditions (4) and (5), the negative power which constitutes thetelephoto type becomes strong too much. Accordingly, the positive powermust also be strengthened and the aberration generated in each lensincreases, and securing of the performance becomes difficult.

[0074] In the image forming optical system, it is preferable to satisfythe following conditions (4′) and (5′).

0.4<f 12/|f 3|<1.3  (4′)

0.7<f/|f 3|<1.5  (5′)

[0075] Furthermore, it is preferable to satisfy the following conditions(4″) and (5″).

0.6<f 12/|f 3|<0.9  (4″)

0.9<f/|f 3|<1.2  (5″)

[0076] In the image forming optical system, the first lens, the secondlens and the third lens are arranged where an aperture stop is arrangedbetween the first lens and the second lens. Here, in order to makemagnification chromatic aberration and distortion small, it is importantto constitute so that an off-axis light flux may pass in point symmetryto the center position of the aperture stop.

[0077] Therefore, in the image forming optical system it is desirable tosatisfy the following condition (6).

−5.0<f 1/f 23<3.0  (6)

[0078] where f1 is the focal length of the first lens and f23 is thetotal focal length of the second lens and the third lens.

[0079] When f1/f23 exceeds the upper limit of the condition (6) orfalling below the minimum limit of the condition (6), magnificationchromatic aberration and distortion are excessively corrected orinsufficiently corrected. Consequently, in both cases, the performanceof circumference becomes worse.

[0080] In the image forming optical system it is preferable to satisfythe following condition (6′).

0<f 1/f 23<1.0  (6′)

[0081] Furthermore, in the image forming optical system is desirable tosatisfy the following condition (6″).

0.4<f 1/f 23<0.7  (6″)

[0082] Moreover, in the image forming optical system, in order tocorrect an axial chromatic aberration it is desirable that achromatismis performed by the whole lenses and the following condition (7) issatisfied.

0.5<(ν1−ν3)/(ν2−ν3)<1.5  (7)

[0083] where ν1 represent the Abbe's number of the first lens, ν2 is theAbbe's number of the second lens and ν3 is the Abbe's number of thethird lens.

[0084] When exceeding the upper limit of the condition (7) or fallingbelow the minimum limit of the condition (7), an axial chromaticaberration is excessively corrected or insufficiently corrected.Consequently, securing of the performance of center portion becomesdifficult. In the image forming optical system, it is preferable tosatisfy the following condition (7′).

0.7<(ν1−ν3)/(ν2−ν3)<1.3  (7′)

[0085] Furthermore, in the image forming optical system it is desirableto satisfy the following condition (7″).

0.8<(ν1−ν3)/(ν2−ν3)<1.2  (7″)

[0086] Meanwhile, in case that CCD is used for an imaging element, thephenomenon so-called shading occurs. This is the phenomenon in which thebrightness of the picture image differs at the center portion of thepicture image and at the circumferential portion of the picture imagewhen an off-axis light flux emanated from an optical system enters intoan image plane.

[0087] On the other hand, if an incident angle to the image plane issmall, the shading problem is reduced. But in this case, the totallength of imaging optical system becomes long.

[0088] Therefore, in the image forming optical system, it is desirableto satisfy the following condition (8).

0.4<EXP/f<1.5  (8)

[0089] Where EXP is the distance to the exit pupil from the image plane,and f is the whole focal length of the image forming optical system as awhole system.

[0090] When EXP/f exceeds the upper limit of the condition (8) the wholelength of the image forming optical system becomes large.

[0091] On the other hand, when EXP/f falls below the minimum limit ofthe condition (8), the angle of incidence to CCD becomes big too much,and the brightness of circumferential portion of the picture imagedecreases.

[0092] In the image forming optical system it is preferable to satisfythe following condition (8′).

0.6<EXP/f<1.3  (8′)

[0093] Furthermore, in the image forming optical system it is desirableto satisfy the following condition (8″).

0.8<EXP/f<1.1  (8″)

[0094] Moreover, in the image forming optical system, it is desirable tosatisfy the following condition (9).

0.55[1/μm]<Fno/P[μm]<2.10[1/μm]  (9)

[0095] Where Fno is F number of the optical system and P is pixel pitchof an imaging element.

[0096] When Fno/P exceeds the upper limit of the condition (9) lightquantity per one picture element becomes small since the optical systembecomes too dark or the pixel pitch of the picture element becomes toosmall. Accordingly, the shutter speed becomes slow, and this may causehand blur and increase of noise by a long time exposure. On the otherhand, when Fno/P falls below the minimum limit of the condition (9), thepixel pitch of the picture element becomes too large and accordinglyimaging data with fine pixel pitch cannot be obtained. In the imageforming optical system it is preferable to satisfy the followingcondition (9′).

0.65[1/μm]<Fno/P[μm]<1.50[1/μm]  (9′)

[0097] Furthermore, in the image forming optical system is preferable tosatisfy the following condition (9″).

0.77[1/μm]<Fno/P[μm]<1.18[1/μm]  (9″)

[0098] Moreover, in the image forming optical system, it is desirable tosatisfy the following condition (10).

0.05<ML/TL<0.35  (10)

[0099] where ML represents the minimum thickness of a plastic lens onthe axis and TL is the whole length of the optical system.

[0100] When ML/TL exceeds the upper limit of the condition (10) theworkability of a glass lens is aggravated, since the thickness of centerof the glass lens cannot be sufficiently secured because the minimumthickness of a plastic lens on the axis is too big to the whole lengthof the optical system. On the other hand, when ML/TL falls below theminimum limit, the productivity is aggravated since plastic resin cannotbe entered smoothly into a formation die because the minimum thicknessof a plastic lens on the axis is too small and accordingly it causesgenerating stress and double refraction as well as more time consumingfor molding.

[0101] In the image forming optical system it is preferable to satisfythe following condition (10′).

0.10<ML/TL<0.27  (10′)

[0102] Furthermore, in the image forming optical system is desirable tosatisfy the following condition (10″).

0.14<ML/TL<0.20  (10″)

[0103] Hereafter, embodiments of the present invention will be explainedusing drawings.

The First Embodiment

[0104]FIG. 1 is a sectional view showing an optical arrangementdeveloped along the optical axis of an image forming optical system inthe first embodiment. FIG. 2A, 2B and 2C are graphs showing sphericalaberration, astigmatism and distortion of an image forming opticalsystem in the first embodiment respectively.

[0105] An image forming optical system of the first embodimentcomprises, in order from an object side, a first lens L1 which ismeniscus lens having positive refractive power and a convex asphericalsurface directed toward an object side, an aperture stop S, a secondlens L2 which is meniscus lens having positive refractive power, doubleaspherical surfaces and a convex surface directed toward the image side,and a third lens L3 having negative refractive power, double asphericalsurfaces where the power of the center portion is negative and the powerof the circumferential portion is positive.

[0106] In FIG. 1, reference symbol I represents an imaging plane of animaging element. On the image plane of the optical system, an imagingelement having 1,300,000 pixels (a pitch of picture element 3.6 μm) in ⅓inches square is arranged.

[0107] Lens data of optical members constituting the image formingoptical system of the first embodiment are listed below.

[0108] In the first embodiment, the first lens L1 and the second lens L2are made of glass and the third lens L3 is made of plastic(polycarbonate). In the lens data of the first embodiment, refractiveindices and Abbe's numbers are values at e-lines.

[0109] When z is taken as the coordinate in the direction of the opticalaxis, y is taken as the coordinate normal to the optical axis, Krepresents a conic constant, and a, b, . . . represent asphericalcoefficients, the configuration of each of the aspherical surface isexpressed by the following equation:

z=(y ² /r)/[1+{1−(1 30 K)(y/r)²}^(1/2) ]+ay ⁴ +by ⁶+. . .

[0110] These symbols are commonly used in embodiments to be describedlater.

[0111] Numerical Data 1

[0112] focal length f=4.6 mm, Fno.=2.8, image height HT=3.0 mm, halffield angle ω=31°. Surface Surface Radius (air space) Refraction No.Curvature Distance Index Abbe's No. Object ∞ ∞ surface 1 aspherical 1.121.5163 64.1 surface [1] 2 8.55 0.44 3 aperture stop 0.77 4 aspherical1.55 1.5891 61.2 surface [2] 5 aspherical 0.22 surface [3] 6 aspherical1.26 1.5839 30.2 surface [4] 7 aspherical 1.66 surface [5] Image plane ∞aspherical surface [1] radius of curvature 2.32 k = 2.1257 × 10⁻² a =2.4932 × 10⁻³ b = 7.0861 × 10⁻⁴ aspherical surface [2] radius ofcurvature −1.80 k = 1.3698 × 10⁺⁰ a = −7.3856 × 10⁻² aspherical surface[3] radius of curvature −1.14 k = −8.3219 × 10⁻¹ a = 2.7759 × 10⁻² b =−1.4018 × 10⁻² aspherical surface [4] radius of curvature −23.77 k =7.6623 × 10⁺¹ a = 4.3204 × 10⁻³ aspherical surface [5] radius ofcurvature 2.70 k = −1.3298 × 10⁺¹ a = −1.5317 × 10⁻² b = 6.0974 × 10⁻⁴

The Second Embodiment

[0113]FIG. 3 is a sectional view showing an optical arrangement,developed along the optical axis of an image forming optical system inthe second embodiment. FIG. 4A, 4B and 4C show spherical aberration,astigmatism and distortion of an image forming optical system in thesecond embodiment respectively.

[0114] An image forming optical system in the second embodimentcomprises, in order from an object side, a first lens L1 which ismeniscus lens having positive refractive power, an aspherical convexsurface directed toward an object side, an aperture stop, a second lensL2 which is meniscus lens having positive refractive power, doubleaspherical surfaces and a convex surface directed toward the image side,and a third lens L3 having negative refractive power, double asphericalsurfaces where the power of the center portion is negative and the powerof the circumferential portion is positive.

[0115] In FIG. 3, reference symbol I represents an imaging plane of animaging element.

[0116] Lens data of optical members constituting the image formingoptical system of the second embodiment are listed below.

[0117] In the second embodiment, the first lens L1 is made of glass andthe second lens L2 and the third lens L3 are made of plastic. In detail,the second lens L2 is made of Zeonex and the third lens L3 is made ofpolycarbonate.

[0118] On the image plane of the optical system, an imaging elementhaving 2,000,000 pixels (a pitch of picture element 3.0 μm ) in ⅓ inchessquare is arranged.

[0119] Numerical Data 2

[0120] focal length f=4.6 mm, Fno.=2.4, image height HT=3.0 mm, halffield angle ω=31°. Surface Surface Radius (air space) Refraction No.Curvature Distance Index Abbe's No. Object ∞ ∞ surface 1 aspherical 1.041.5831 59.4 surface [1] 2 8.55 0.44 3 aperture stop 0.76 4 aspherical1.23 1.5256 56.4 surface [2] 5 aspherical 0.10 surface [3] 6 aspherical1.46 1.5839 30.2 surface [4] 7 aspherical 1.56 surface [5] Image plane ∞aspherical surface [1] radius of curvature 2.19 k = 1.0272 × 10⁻¹ a =1.9376 × 10⁻³ b = 7.2822 × 10⁻⁴ aspherical surface [2] radius ofcurvature −1.50 k = 1.0172 × 10⁺⁰ a = −8.5104 × 10⁻² aspherical surface[3] radius of curvature −1.02 k = −7.7974 × 10⁻¹ a = 3.1554 × 10⁻² b =−2.0397 × 10⁻² aspherical surface [4] radius of curvature −25.93 k =1.1898 × 10⁺² a = 2.7400 × 10⁻³ aspherical surface [5] radius ofcurvature 2.95 k = −1.5198 × 10⁺¹ a = −2.3321 × 10⁻² b = 9.9319 × 10⁻⁴

The Third Embodiment

[0121]FIG. 5 is a sectional view showing an optical arrangement,developed along the optical axis, of an image forming optical system inthe third embodiment. FIG. 6A, 6B and 6C show spherical aberration,astigmatism and distortion of an image forming optical system in thethird embodiment respectively.

[0122] An image forming optical system in the third embodimentcomprises, in order from an object side, a first lens L1′ which ismeniscus lens having positive refractive power, double asphericalsurfaces and a convex surface directed toward an object side, anaperture stop, a second lens L2 which is meniscus lens having positiverefractive power, double aspherical surfaces and a convex surfacedirected toward the image side, and a third lens L3 having negativerefractive power, double aspherical surfaces where the power of thecenter portion is negative and the power of the circumferential portionis positive. In FIG. 5, reference symbol I represents an imaging planeof an imaging element.

[0123] Lens data of optical members constituting the image formingoptical system of the third embodiment are listed below.

[0124] In the third embodiment, all lenses are made of plastic.

[0125] In detail, the first lens and the second lens are made of Zeonexand the third lens is made of polycarbonate.

[0126] On the image plane of the optical system, an imaging elementhaving 3,000,000 pixels (a pitch of picture element 2.4 μm) in ⅓ inchessquare is arranged.

[0127] Numerical Data 3

[0128] focal length f=4.5 mm, Fno.=2.8, image height HT=3.0 mm, halffield angle ω=34°. Surface Surface Radius (air space) Refraction No.Curvature Distance Index Abbe's No. Object ∞ ∞ surface 1 aspherical 1.061.5256 56.4 surface [1] 2 aspherical 0.46 surface [2] 3 aperture stop0.66 4 aspherical 1.22 1.5256 56.4 surface [3] 5 aspherical 0.10 surface[4] 6 aspherical 1.45 1.5839 30.2 surface [5] 7 aspherical 0.14 surface[6] 8 ∞ 1.51 Image plane ∞ aspherical surface [1] radius of curvature2.04 k = 3.5240 × 10⁻¹ a = 3.8084 × 10⁻⁴ b = 5.3270 × 10⁻⁴ asphericalsurface [2] radius of curvature 5.60 k = 2.9258 × 10⁺⁰ a = 5.6408 × 10⁻³aspherical surface [3] radius of curvature −1.36 k = 8.2622 × 10⁻¹ a =−9.7733 × 10⁻² b = 1.4302 × 10⁻² aspherical surface [4] radius ofcurvature −0.97 k = −7.2104 × 10⁻¹ a = 3.7885 × 10⁻² b = −2.0112 × 10⁻²aspherical surface [5] radius of curvature −58.12 k = 5.7689 × 10⁺² a =3.4088 × 10⁻³ aspherical surface [6] radius of curvature 2.94 k =−1.6741 × 10⁺¹ a = −2.4078 × 10⁻² b = 1.3122 × 10⁻³

[0129] The Fourth Embodiment

[0130]FIG. 7 is a sectional view showing an optical arrangement,developed along the optical axis of an image forming optical system inthe fourth embodiment. FIG. 8A, 8B and 8C show spherical aberration,astigmatism and distortion of an image forming optical system in thefourth embodiment respectively.

[0131] An image forming optical system in the fourth embodimentcomprises, in order from an object side, a first lens L1′ which ismeniscus lens having positive refractive power, double asphericalsurfaces and a convex surface directed toward an object side, anaperture stop, a second lens L2 which is meniscus lens having positiverefractive power, double aspherical surfaces and a convex surfacedirected toward the image side, and a third lens L3 having negativerefractive power, double aspherical surfaces where the power of thecenter portion is negative and the power of the circumferential portionis positive.

[0132] In FIG. 7, reference symbol I represents an imaging plane of animaging element.

[0133] Lens data of optical members constituting the image formingoptical system of the third embodiment are listed below.

[0134] In the fourth embodiment, all lenses are made of plastic.

[0135] In detail, the first lens and the second lens are made of Zeonexand the third lens is made of polycarbonate.

[0136] On the image plane of the optical system, an imaging elementhaving 2,000,000 pixels (a pitch of picture element 3.0 μm ) in ⅓ inchessquare is arranged.

Numerical Data 4

[0137] focal length f=4.65 mm, Fno.=2.8, image height HT=3.0 mm, halffield angle ω=33°. Surface Surface Radius (air space) Refraction No.Curvature Distance Index Abbe's No. Object ∞ ∞ surface 1 aspherical 1.051.5256 56.4 surface [1] 2 aspherical 0.45 surface [2] 3 aperture stop0.64 4 aspherical 1.56 1.5256 56.4 surface [3] 5 aspherical 0.10 surface[4] 6 aspherical 1.88 1.5839 30.2 surface [5] 7 aspherical 1.33 surface[6] 8 ∞ 1.51 Image plane ∞ aspherical surface [1] Radius of curvature2.05 k = 4.6507 × 10⁻¹ a = 3.7625 × 10⁻⁴ b = 8.1987 × 10⁻⁴ asphericalsurface [2] radius of curvature 5.69 k = 3.5391 × 10⁺⁰ a = 1.1185 × 10⁻²aspherical surface [3] radius of curvature −1.68 k = 1.4236 × 10⁺⁰ a =−4.7874 × 10⁻² b = −3.4864 × 10⁻² aspherical surface [4] radius ofcurvature −1.12 k = −6.0437 × 10⁻¹ a = 4.2650 × 10⁻² b = −1.0446 × 10⁻²aspherical surface [5] radius of curvature −34.06 k = 2.2622 × 10⁺² a =−1.1040 × 10⁻³ aspherical surface [6] radius of curvature 2.68 k =−1.1850 × 10⁺¹ a = −1.6923 × 10⁻² b = 3.9551 × 10⁻⁴

[0138] In the embodiments mentioned above, all of lenses in the imageforming optical system are made of plastic. However, all of the lensescan be constituted with glass. For example, if the lenses areconstituted with glass having refractive index higher than those used inthe embodiments, it goes without saying that higher performance can beachieved.

[0139] If special low dispersion glass is used for lenses used in theimage forming optical systems of the embodiments mentioned above, it iseffective for correction of the chromatism. In particular, it isdesirable to constitute a lens by plastic, because degradation of theperformance due to environment change can be reduced by using lowhygroscopic material (for example, Zeonex etc., product of Nippon ZeonCo.).

[0140] In each embodiment mentioned above, a flare cut stop may be usedinstead of an aperture stop in order to cut an unnecessary light ofghost, flare and the like. This flare cut aperture may be arranged inany place which is either in front of the first lens, between the firstlens and the aperture stop, between the aperture stop and the secondlens, between the second lens and the third lens, or between the thirdlens and the image surface side in the embodiment.

[0141] The flare cut stop may be constituted so as to cut flare light bya frame, and it may be constituted with another member. Also, it ispossible to constitute a flare cut aperture by printing, painting andgluing a seal and the like, directly to the image forming opticalsystem. As to the shape of the stop, any type of shape formed by such asa circle, an ellipse, a rectangle, a polygon and a scope surrounded by afunction curve can be also used. By arranging a flare stop, it ispossible to cut not only detrimental luminous flux but also luminousflux of the coma flare and the like around the picture plane.

[0142] Moreover, in order to reduce ghost and flare, coating forpreventing of reflection can be made to each lens. In such case, it isdesirable to carry out multiple coating since the ghost and the flarecan be efficiently reduced. Furthermore, infrared cut coating can bealso made to surfaces of a lens and a cover glass and the like.

[0143] Furthermore, in the image forming optical system of eachembodiment mentioned above, focusing can be carried out for adjustingthe focus. In such case, the whole lens system can be moved outward forfocusing, or a part of lenses can be moved outward or inward forfocusing.

[0144] In the image forming optical system of each embodiment mentionedabove, decrease of the brightness around circumferential portion of apicture image plane can be reduced by shifting a micro lens of CCD. Forexample, the design of the micro lens of CCD may be changed according tothe incident angle of the light at each image height. Correction ofdecreased quantity of the brightness around circumferential portion of apicture image plane can be carried out by image processing.

[0145] Although illustration is not shown, the image forming opticalsystem is suitable for an optical apparatus such as camera, cellularphone, portable type information entry terminal and the like using filmand CCD as a recording part. Therefore, an optical apparatus having theoptical system mentioned above is also included as the presentinvention.

[0146] Next, numerical values calculated by parameters in conditionsmentioned above concerning each embodiment are shown in the followingtable 1. TABLE 1 first second third fourth embodiment embodimentembodiment embodiment Φ_(m)/Φ_(p) −0.15 −0.28 −0.38 −0.04 (r1r + r2f)/0.65 0.59 0.61 0.54 (r1r − r2f) r1f/f 0.50 0.48 0.45 0.44 f12/|f3| 0.800.76 0.70 0.84 f/|f3| 1.14 1.04 0.95 1.12 f1/f23 0.66 0.48 0.61 0.44 (ν1− ν3)/ 1.09 1.11 1.00 1.00 (ν2 − ν3) EXP/f 1.00 0.91 0.94 0.86 Fno/P0.78 0.80 1.17 0.93 ML/TL 0.18 0.19 0.16 0.15

[0147]FIG. 9 shows outlined construction of an embodiment of anelectronic instrument concerning the present invention. Here, FIG. 9A isa front view showing appearance of the cellular phone and FIG. 9B is arear view showing the same. In FIG. 9, reference numeral 1 represents anantenna for transmitting and receiving an electric wave, referencenumeral 2 represents a display portion such as LCD, reference numeral 3is a speaker portion from which a user catches sound, reference numeral4 is an operation portion, reference numeral 5 is a microphone portion,reference numeral 6 is an image forming optical system portion includingan image forming optical system of the present invention, which isarranged at the opposite side to the said portions and reference numeral7 is a battery and reference numeral 8 is a back side monitor.

[0148]FIG. 10 shows outlined construction of a digital camera in whichan image forming optical system according to the present invention isused in a photographing optical system. FIG. 10A is a front perspectiveview showing appearance of the digital camera and FIG. 10B is a rearperspective view of the same. In FIG. 10, reference numeral 11represents a photographing optical system having photographing opticalpath 12, reference numeral 13 is a finder optical system with an opticalpath 14 for finder, reference numeral 15 is a shutter button, referencenumeral 16 is a flush lump and reference numeral 17 is a monitor withliquid crystal display.

[0149] When the shutter button 15 arranged on the camera is pressed, inresponding such action, photographing is carried out via thephotographing optical system 11.

What is claimed is:
 1. An image forming optical system comprising: inorder from an object side, a first lens which is meniscus lens havingpositive refractive power and a convex surface directed toward an objectside, an aperture stop, a second lens which is meniscus lens havingpositive refractive power and a convex surface directed toward theobject side, and a third lens having negative refractive power.
 2. Animaging optical system according to claim 1, wherein at least one ofsurfaces of the third lens is aspherical and the following condition issatisfied: −2.0<Φm/Φp<0where Φm represents the power of the third lensat the position of the maximum light height and Φp represents the powerof the third lens at the position of the near axis.
 3. An imagingoptical system according to claim 1, satisfying the following condition:−2.0<(r 1 r+r 2 f)/(r 1 r−r 2 f)<1.0where r1r represents the radius ofcurvature of the first lens at the image side and r2f is the radius ofcurvature of the second lens at the object side
 4. An imaging opticalsystem according to claim 1, satisfying the following condition: 0.1 <r1 f/f<1.0where r1f represents the radius of curvature of the first lensat the object side and f is the focal length of the whole image formingoptical system.
 5. An imaging optical system according to claim 1,satisfying the following conditions: 0.2<f 12/|f 3<1.50.5<f/|f3|<2.0where f12 represents total focal length of the first lens and thesecond lens, f3 represents the focal length of the third lens and frepresents the focal length of the whole optical system.
 6. An imagingoptical system according to claim 1, satisfying the following condition:−5.0<f 1/f 23<3.0where f1 represents the focal length of the first lensand f23 represents the total focal length of the second lens and thethird lens.
 7. An imaging optical system according to claim 1,satisfying the following condition: 5.0<(ν1−ν3)/(ν2−ν3)<1.5where ν1represents the Abbe's number of the first lens, ν2 represents that ofthe second lens and ν3 is that of the third lens.
 8. An imaging opticalsystem according to claim 1, satisfying the following condition:0.4<EXP/f<1.5where EXP represents the distance of the exit pupil from animage plane and f is the whole focal length of the image forming opticalsystem.
 9. An imaging optical system according to claim 1, satisfyingthe following condition: 0.55(1/μm)<Fno/P(μm)<2.10(1/μm) where Fnorepresents the F number fully opened and P represents the pixel pitch ofan imaging element which has an imaging plane at the image formingposition of the image forming optical system.
 10. An imaging opticalsystem according to claim 1, satisfying the following condition:0.05<ML/TL<0.35where TL represents whole length of the image formingoptical system and ML represents the minimum thickness on the axis of aplastic lens constituting the image forming optical system.
 11. Anelectric device equipped with the image forming optical system of claim1.